Tunable Emission Phosphor Ca0.75Sr0.2Mg1.05(Si2O6):Eu2+, Mn2+:

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A Tunable Emission Phosphor Ca0.75Sr0.2Mg1.05(Si2O6):Eu2+, Mn2+: Luminescence and Mechanism of Host, Energy Transfer of Eu2+#Mn2+, Eu2+#Host and Host#Mn2+ Yuansheng Sun, Pan-Lai Li, Zhijun Wang, Jinge Cheng, Zhenling Li, Chao Wang, Miaomiao Tian, and Zhiping Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05993 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 28, 2016

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The Journal of Physical Chemistry

A Tunable Emission Phosphor Ca0.75Sr0.2Mg1.05(Si2O6):Eu2+, Mn2+: Luminescence and Mechanism of Host, Energy Transfer of Eu2+→Mn2+, Eu2+→Host and Host→Mn2+ Yuansheng Sun, Panlai Li*, Zhijun Wang*, Jinge Cheng, Zhenling Li, Chao Wang, Miaomiao Tian, Zhiping Yang College of Physics Science & Technology, Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University, Baoding 071002, China ABSTRACT: A series of Ca0.75Sr0.2Mg1.05(Si2O6) (CSMS):xEu2+, yMn2+ phosphors were synthesized by a high temperature solid state reaction technique in a reducing atmosphere (1250℃, 4 h). Under excitation of 270 nm, the emission spectrum of CSMS host is characterized by a broad band with two distinct emission peaks (453 nm and 580 nm), tailing on the energy side from 350 nm to 800 nm approximately. Thus, there is an orange-yellow emission in CSMS host. The different trap depths of thermoluminescence spectra (E1=0.6453 eV, E2=0.8271 eV) are measured and these traps will fade away when Eu2+ or Mn2+ ions are doped. In addition, the blue-shift, redshift and intensity changes of temperature dependence emission spectra in Ca0.75Sr0.2Mg1.05(Si2O6) host are observed with higher temperature. For Eu2+ single doped CSMS, which has an obvious absorption in the near-ultraviolet and blue region with a wide band of excitation spectrum ranging from 200 to 450 nm monitored at 453 nm. And the emission spectrum presents an obvious redshift phenomenon. Mn2+ single-doped CSMS presents the weak emission intensity at 680 nm with the excitation of 415 nm. For Eu2+ and Mn2+ co-doped, the energy transfer of Eu2+→Mn2+, Eu2+→host and host→Mn2+ can be demonstrated, and there should be a blue-white emission in CSMS:0.03Eu2+, 0.02Mn2+ with CIE (0.2682, 0.2056). In short, the CSMS host presents a good application prospect in near ultraviolet trichromatic field of white LEDs, and the luminescence properties of CSMS:xEu2+, yMn2+ phosphors are particularly worthy of academic investigation. 1. INTRODUCTION Rare earth elements and transition metal ions act as important activators doped in phosphors for application in modern lighting and display fields due to their abundant emission colors to achieve spectra conversion.1,2 Especially, Eu is of great potential for activator because of Eu2+ can function as an emission center in the host lattices.3-6 Besides, the Eu2+ ions are sensitive to the crystal field and covalence since its 4f-5d transition is spin-allowed, which generally presents superior

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absorption bands in the spectral region of 250 - 450 nm and broad emission spectra from blue to red region in Eu2+ doped phosphors, matching well with the ultraviolet (UV) and near-UV LED chips.7,8 In addition, as a transition metal ion, Mn2+ has a good stability and has been found applications in many phosphors, and its properties have been most widely studied.9-11 Mn2+activated luminescence materials have been known to show wide-ranging emission from 500 to 700 nm, depending upon the crystal field and the coordination number (CN) of the host materials, which often can produce orange or red color based on its spin-forbidden 3d-3d absorption transitions, thus enhancing its weak emission and generating tunable color attributed to the energy transfer effect from Eu2+ to Mn2+ ions.12-14 Therefore, many investigations have been devoted to them,

such

as

Na3(Y,

Sc)Si3O9:Eu2+,

Li2Ca2Si2O7:Eu2+,

NaxCa1-xAl2-xSi2+xO8:Eu2+,

Ba3Si6O12N2:Eu2+, Sr2−xSi5N8:Eu2+, Ca1.5Ba0.5Si5N6O3:Eu2+, Ba9Sc2Si6O24:Ce3+, Eu2+, Mn2+, KCaY(PO4)2:Eu2+, Mn2+, Ba3MgSi2O8:Eu2+, Mn2+, Ba1.3Ca0.7SiO4:Eu2+, Mn2+.15-24 In other words, the research of Eu2+/Mn2+ co-doping based on the energy transfer from Eu2+ to Mn2+ is a very prevalent and important way in the field of phosphors. But, the energy transfer effect from Eu2+ to Mn2+ is not obvious because of the inverse bottleneck effect of Mn2+. In addition, according to the previously reported, the energy transfer efficiency of Eu2+→ Mn2+ is also linked with the host. Hence, we are considering that whether we can introduce the energy of the host based on energy transfer between Eu2+ and Mn2+. This is very interesting. Silicate has a significant absorption in near-UV and blue region, and has the advantages of high quantum

efficiency,

low

production

cost

and

simple

synthesis

conditions.2,25

Ca0.75Sr0.2Mg1.05(Si2O6), as a novel host compound, has the monoclinic structure and has never been reported in the field of phosphors. The crystal structure of the Ca0.75Sr0.2Mg1.05(Si2O6) compound was first reported by Benna, P. et al in 1987.26 According to previously reported about host luminescent materials, such as Ca4GdO(BO3)3, α-Zn3(PO4)2:Mn2+, K+ and Zn2GeO4.27-29 Ca0.75Sr0.2Mg1.05(Si2O6) compound, as a novel host, attracts our strong interest. We have been synthesized the Ca0.75Sr0.2Mg1.05(Si2O6) host by the high temperature solid-state method. Under the excitation of 270 nm, for CSMS host, the emission spectra with two different main peaks at 453 and 580 nm present a broad emission band from 350 to 800 nm, which results in an orangeyellow emitting. Furthermore, the substitution effects of Ca2+ on the crystal structure and


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luminescence properties of Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+ phosphors are investigated, which is expected to appear the white light used in LEDs according to the principle that yellow and blue to produce white light.21,22 The luminescence properties of Ca0.75-xSr0.2Mg1.05-y(Si2O6):xEu2+, yMn2+ phosphors are also studied because of the energy transfer among Eu2+, Mn2+ and host. Furthermore, the different trap depths of thermoluminescence spectra are measured and these traps will fade away when Eu2+ or Mn2+ ions are doped, and this is also very interesting and attractive. The detailed studies of the synthesis process and luminescent property of CSMS host and CSMS:xEu2+, yMn2+ phosphors are shown in this paper, anyway, the results will provide a new idea or method for future researches to introduce the energy of host based on energy transfer between Eu2+ and Mn2+. What is more, the CSMS host represents a good application prospect in near ultraviolet trichromatic field of white LEDs.30,31 2. EXPERIMENTAL 2.1. Materials and preparation. Ca0.75Sr0.2Mg1.05(Si2O6) host was calcined by the hightemperature solid-state reaction method in the carbon dust reductive atmosphere and air with CaCO3 (A.R.), SrCO3 (A.R.), MgO (A.R.) and SiO2 (A.R.) raw materials at 1250℃ for 4 h, respectively. The specific chemical equations are as follows: 0.75CaCO3+0.2SrCO3+1.05MgO+2SiO2→Ca0.75Sr0.2Mg1.05(Si2O6) (in the carbon dust reductive atmosphere and in air) However, the Ca0.75Sr0.2Mg1.05(Si2O6) host can be synthesized and emit light only in the carbon dust reductive atmosphere since the oxygen vacancies can appear easily in the atmosphere.1 Furthermore, a series of phosphors with composition Ca0.75-xSr0.2Mg1.05-y(Si2O6):xEu2+, yMn2+ were synthesized by the high-temperature solid-state reaction method under reductive atmosphere. And simply put, Eu2O3 (99.99%) and MnCO3 (A.R) were added into the matrix materials to synthesize CSMS:xEu2+, yMn2+ phosphors. The specific chemical equations and the synthetic conditions are expressed as follows: (0.75-x)CaCO3+0.2SrCO3+1.05MgO+2SiO2→Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+ (x=0.001-0.08, in the carbon dust reductive atmosphere) 0.75CaCO3+(0.2-x)SrCO3+1.05MgO+2SiO2→Ca0.75Sr0.2-xMg1.05(Si2O6):xEu2+ (x=0.001-0.08, in the carbon dust reductive atmosphere)



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0.75CaCO3+0.2SrCO3+(1.05-y)MgO+2SiO2→Ca0.75Sr0.2Mg1.05-y(Si2O6):yMn2+ (y=0.01-0.30, in the carbon dust reductive atmosphere) (0.75-0.03)CaCO3+0.2SrCO3+(1.05-y)MgO+2SiO2→CSMS:0.03Eu2+, yMn2+ (x=0.03, y=0.01-0.20; in the carbon dust reductive atmosphere) (0.75-x)CaCO3+0.2SrCO3+(1.05-0.25)MgO+2SiO2→CSMS:xEu2+, 0.25Mn2+ (x=0.001-0.12, y=0.25; in the carbon dust reductive atmosphere) Then, the raw materials were weighed according to the given stoichiometric proportion, and mixed thoroughly by grinding them in an agate mortar, after which the mixture was shifted to the crucible and transformed to the tube furnace to calcine at 1250℃ for 4 h in the carbon dust reductive atmosphere. Finally, after the samples were furnace-cooled to room temperature for a period of time, and ground again into powders to future study. 2.2. Characterization and measurement. In this research, all measurements were conducted using the finely ground powder. The Phase formation of samples were examined by X-ray powder diffraction (XRD) performed on a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15405 nm), operating at 40 mA, 40 KV. Step length and diffraction range were 0.05°, and 20°-80°, respectively. The excitation and emission spectra (PLE and PL), for this research, were measured via the Hitachi F-4600 fluorescence spectrophotometer using a 450 W Xe lamp as the excitation source, with a scanning wavelength from 200 to 800 nm, scanning at 240 nm/min. The temperature-dependent luminescence properties were measured on the same spectrophotometer which was assembled with a computer-controlled electric furnace and a selfmade heating attachment. Luminescence decay curves of CSMS and Mn2+ were obtained using a HORIBA FL-1057 equipped with a 450 W Xe lamp as an excitation source and that of Eu2+ were recorded with the 370 nm pulse laser radiation (nano-LED) at room temperature. Furthermore, diffuse reflection spectra on the phosphors were surveyed on a Hitachi U-4100 machine, at a scanning wavelength range of 200–800 nm. Quantum yields (QYs) of phosphors were obtained directly by the absolute PL quantum yield measurement system (HORIBA, FL-1057). In addition, scanning electron micrograph (SEM) images and electron-dispersive X-ray (EDX) were obtained using a Nova Nano SEM 650 LA. And the thermoluminescence spectra of samples were measured using a FJ-427A1 TL dosimeter with a fixed heating rate of 1 ℃/s within the range 30-300℃.


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Finally, the fluorescent images in the paper were photographed with SLR camera of Canon under a dark environment. It is no doubt that all these above measurements were performed at room temperature. 3. RESULTS AND DISCUSSION

Raw data Calculated line Background line Differences Bragg positions

20

2

χ = 7.318

CSMS Host

30

40

50

(b)

Rp= 8.09% Rwp= 11.28%

60

70

Raw data Calculated line Background line Differences Bragg positions

Intensity(a.u.)

(a) Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

2θ (degree)

20

Rp= 8.02% Rwp= 10.90% χ2= 6.508 2+

2+

CSMS:3%Eu ,2%Mn

30

40

50

60

70

80

2θ (degree)

Figure 1. (a) Rietveld refinement of powder XRD profiles of the representative CSMS host, (b) CSMS:3%Eu2+, 2%Mn2+.

Table 1. Crystallographic data and details in the data collection and refinement parameters for Ca0.75Sr0.2Mg1.05(Si2O6) and CSMS:3%Eu2+, 2%Mn2+. Sample

Ca0.75Sr0.2Mg1.05(Si2O6)

Space group

C12/c1

Symmetry

monoclinic

2θ-interval ( °)

CSMS:3%Eu2+, 2%Mn2+

20-80

C12/c1 monoclinic 20-80

a (Å)

9.746062

9.767794

b (Å)

8.944250

8.961537

c (Å)

5.248386

5.263494

V (Å3)

440.094

443.101

Z

4

4

α (°)

90.00

β (°)

105.856

105.904

Rp

8.09%

8.02%

Rwp

11.28%

10.90%

7.318

6.508

χ2


90.00


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3.1. Crystal structure and phase formation. Figure 1a and Figure 1b show the Rietveld analysis of powder XRD profiles of the representative CSMS host and CSMS:3%Eu2+, 2%Mn2+ conducted by the general structure analysis system (GSAS) method, in which the red solid lines, black crosses, olive lines and wine red bars are calculated patterns, experimental patterns, differences and Bragg position, respectively. The differences between the calculated and experimental results are expressed with olive bars, which are depicted between the background line (blue line) and the Bragg reflection line. The initial structure model of Ca0.75Sr0.2Mg1.05(Si2O6) (ICSD #68180) crystallizing in monoclinic system with space group C12/c1 was used to refine the above samples.32 The refined results indicate that all atom coordinates, fraction factors as well as thermal vibration parameters are fitted well under the reflection conditions, Rp = 8.09%, 8.02%, Rwp = 11.28%, 10.90% and χ2= 7.318, 6.508, respectively, which illustrates that there is no detectable impurity phase observed in these obtained samples. The more detailed information of refinement can be found in Table 1. For CSMS:3%Eu2+, 2%Mn2+, by contrast, the crystal cell parameters are slightly bigger, which is reasonable and consistent with ionic radius. The chemical composition of CSMS:3%Eu2+ and CSMS:3%Eu2+, 2%Mn2+ compounds are further determined by EDX, as shown in Figure 2a and Figure 2b, respectively. The signals of calcium (Ca), strontium (Sr), magnesium (Mg), oxygen (O), silicon (Si), europium (Eu) and manganese (Mn) suggest the presence of the corresponding element in these products, in a word, all the doping elements can be found as expected in the as-prepared samples.

(a)

(b)

Figure 2.（a）EDX (energy-disperse X-ray analysis) of CSMS:3%Eu2+; (b) EDX (energy-disperse X-ray analysis) of CSMS:3%Eu2+, 2%Mn2+.


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Figure 3. Crystal structure of Ca0.75Sr0.2Mg1.05(Si2O6) host.

The Ca0.75Sr0.2Mg1.05(Si2O6) compound has the structure of monoclinic system with cell parameters a = 9.750 Å, b = 8.953 Å, c = 5.249 Å, Z = 4, V = 440.49 Å3, α = 90°, β = 105.73° and γ= 90°, and these parameters provide the best agreement with refinement data (a = 9.746062 Å, b = 8.944250 Å, c = 5.248386 Å). In addition, Ca2+, Sr2+ and Mg2+ ions are distributed in the same site at 4e while the Si and O atoms are fixed on 8f site. In particular, Ca and Sr atoms have the exactly same position with the spatial coordinates x=0, y=0.3015, z=0.25 in this crystal structure. The detailed crystal structure of Ca0.75Sr0.2Mg1.05(Si2O6) is presented in Figure 3. One can note that two obvious atomic groups: MgO (CaO/SrO) and SiO3 are shown in Figure 3a, and the latter is more likely to result in fluorescence because of oxygen vacancy.27,33,34 The more specific atomic structure can be seen in Figure 3b, the acidic chemical group in which a silicon atom is bound to three oxygen atoms (O1, O2, O3) with the unequal bond length (0.1604 nm, 0.1590 nm, 0.1668 nm), respectively.

Figure 4. EDX (energy-disperse X-ray analysis) of Ca0.75Sr0.2Mg1.05(Si2O6) compound.



e

c

x=0, y=0.25

b

x=0.03, y=0 Ca0.75Sr0.2Mg1.05(Si2O6)

a

(1)

Host

40

50

60

2-Theta(Deg.)

70

2θ=35.52 2θ=35.85

d

B=0.30 D=45.84

2θ=35.64

B=0.36

2θ=35.27

D=61.06 B=0.27 D=50.02 B=0.33

2θ=35.70

c

b

a

(2)

JCPDS 80-0388 30

B=0.31

D=55.04

Intensity(a.u.)

x=0.03, y=0.15

d

20

D=53.22

x=0.03, y=0.25

e

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

34.0

34.5

35.0

35.5

36.0

36.5

37.0

2-Theta(Deg.)

Figure 5 (1) The powder XRD patterns of CSMS:xEu2+, yMn2+ (host; 3%Eu2+; 25%Mn2+; 3%Eu2+,15%Mn2+; 8%Eu2+, 25%Mn2+, respectively). As well as the standard reference of Ca0.75Sr0.2Mg1.05(Si2O6) (JCPDS 80-0388). (2) Estimated crystallite size of as-formed the representative samples.

The EDX of CSMS host is given in Figure 4. The signals of calcium (Ca), strontium (Sr), magnesium (Mg), oxygen (O), silicon (Si) suggest the presence of the corresponding element in this product, in a word, all the doping elements can be found as expected in the as-prepared sample. The XRD patterns of Ca0.75-xSr0.2Mg1.05-y(Si2O6):xEu2+, yMn2+ (x=y=0; x=0.03, y=0; x=0,

y=0.25; x=0.03, y=0.25; x=0.08, y=0.25) are displayed in the Figure 5.1. All the diffraction peaks and profiles matched well with those of Ca0.75Sr0.2Mg1.05(Si2O6) phase according to the standard reference of JCPDS 80-0388 and no any traces of impurity phases are observed, indicating that Eu2+ and Mn2+ can be well doped into this host and little impact on crystal structure.12 For Ca0.75Sr0.2Mg1.05(Si2O6) host, there is no shift everywhere compared to the standard reference of JCPDS 80-0388, that is, the host was synthesized according to determining ratios of Ca/Sr/Mg. As shown in Figure 5.1 (a-e), however, the main discrepancy is the intensity ratio of the XRD spectra patterns, and we can find that altitude intercepts of the two peaks circled in the profiles ( 2θ = 35o ) have an obvious change. The small peak on the right became higher and narrower while the peak on the left got shorter, which can be seen in Figure 5.1 compared with the chart a and b. Inversely, mixed Mn2+ into the host can give rise to the intensity of these two small peaks become weaker and a little wider (chart c, Figure 5.1). Whilst, a similar trend can observed in Eu2+/Mn2+ co-doped samples, which are displayed on the chart d and e (Figure 5.1). Hence, there are two possible explanations for these findings. One is that Eu2+ and Mn2+ ions have different signal intensity for CSMS samples in the same test conditions. Another might be due to different


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homogenous dispersion of the crystals or different crystallization of the samples because the intensity of diffraction peak in XRD spectra pattern generally represents the crystallinity of the compounds.35 Owing to the different ionic radius among Ca2+ (r = 1.18 Å for CN = 9 and r = 1.06 Å while CN = 7), Sr2+ ( r = 1.13 Å for CN = 6, r = 1.21 Å for CN = 7 and r = 1.25 Å for CN = 8 ), Mg2+ (r = 0.72 Å for CN = 6 and r = 0.66 Å when CN =5), Eu2+ (r = 1.25 Å for coordinate number (CN) = 8 and r = 1.17 Å for CN = 6) and Mn2+ (r = 0.67 Å for CN = 6, r = 0.66 Å for CN = 4), the diffraction peaks shift to lower angles with doping of Eu2+ and Mn2+ (shown in the Figure 5.1). This shift can be explained by the Bragg’s equation ( θ λ).32 But, the special phenomenon of chart d may be ascribed to a part of Eu2+ substitute for Sr2+. what's more, it may indicate that Eu2+ have a greater effect on the migration of peaks than Mn2+ ions compared chart a, b, c, and e. Appreciable broadening is observed in the X-ray diffraction patterns when the particle size is less than 100 nm and the observed line broadening can be used to calculate the average particle size of the phosphor.36 The average particle size is estimated using Debye Scherrer’s equation37,38 λ

θ

(1)

Figure 6. The scanning electron micrograph (SEM) images of CSMS, CSMS:3%Eu2+, CSMS:25%Mn2+ and CSMS:3%Eu2+, 25%Mn2+.

where D is the average grain size of the material, K is the scherrer's constant (k=0.89), λ is the wavelength of X-rays (0.154056), B is the FWHM (red), θ is the Bragg’s angle (rad). More specific information is shown clearly in Figure 5.2, in which the grain size of samples changed frequently from 45.84 to 61.06 nm. The same phenomenon is also shown in plate a to e.



Compared with Eu2+, therefore, the conclusion that Mn2+ ions can lead to the particle size of these phosphor smaller is validated once again. Furthermore, the SEM images of different CSMS:xEu2+, yMn2+ samples can be obtained from Figure 6. In these images, the particle size of sample is fixed on 10 um to observe the crystallinity of samples. And the conclusion seems to more support the latter speculation about the different crystallization mentioned above. 3.2. Luminescence properties of Ca0.75Sr0.2Mg1.05(Si2O6). Emission and excitation spectra of CSMS host are presented in Figure 7a and Figure 7b. Under excitation of 270 nm, the emission spectrum is characterized by a broad band with two distinct emission peaks (453 nm and 580 nm), tailing on the energy side from 350 nm to 800 nm approximately, which resulted in an orangeyellow emitting of host under the irradiation of 254 nm UV lamp shown in Figure 7a. However, there is only one emission ranging from 400 to 600 nm with main peak at 453 nm under the excitation of 350 nm. Furthermore, the different excitation spectra are monitored at 580 nm and 453 nm, respectively. Both of that present a wide band ranging from 200 to 450 nm, which indicates the excitation is corresponded to the emission. Relying on obtained results, it can be assumed that the luminescence properties of the CSMS host is associated with oxygen vacancy defect, and two disparate emission peaks should correspond to different trap depths.5,39,40

PLE

200

PL

300

(b)

λex=270nm

λem=580nm

(a)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400

500

600

λem=453nm

250

300

800

λex=350nm

PL

PLE 200

700

350

400

450

500

550

600

Wavelength(nm) Figure 7. Excitation and emission spectra of Ca0.75Sr0.2Mg1.05(Si2O6 ) host with excitation wavelength at 270 ,350 nm and emission at 453, 580 nm, respectively .

The temperature dependence emission spectra of the Ca0.75Sr0.2Mg1.05(Si2O6) host upon 270 nm excitation are depicted in Figure 8a. The intensity is falling between 20℃ and 50℃ observed the


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emission at 453 and 580 nm, which may be a result of luminescence from very shallow levels. For this level, the temperature of thermal equilibrium might be less than 50℃. The presence of such levels will have only a small effect on the recombination kinetics as thermalization reduces the net capture rate to an insignificant level at temperatures above 50℃.41,42 Below 50℃, they can result in appreciable carrier trapping and hence contribute to the slow recombination rate at low temperature. In addition, the intensity ratio of 453 nm to 580 nm becomes weaker with the increasing temperature. The intensity emitted at 580 nm should keep getting higher after the temperature at 140℃ as shown in Figure 8b. The reason of these phenomena might be caused by deep energy level which plays an increasingly significant role. Furthermore, it could be invoked to explain by the luminescent process shown in Figure 9 (Process 5-8). For CSMS host, It is noteworthy at this point that the emission peak at 453nm gradually shifts to short wavelength with the increasing of operated temperature, which is dominated by the thermally active phononassisted excitation from the excited states of the lower-energy emission band to the higher-energy emission band in the excited states of host (process 1-4). To the contrary, the peak at 580 nm shifts toward long wavelength gradually, and it might be accounted for the shifting of the luminescence center because of the different trap depths (process 5-8).41 Therefore, it shows that the luminescence has mainly two emission peaks: one peak corresponds to the red shift relative to the

(a) 580nm

453nm

Intensity(a.u.)

central transition frequency and the other to the blue shift.

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

λem=453nm

(b)

50

λem=580nm

140 oC

100

150

200

250

Temperature (oC )

400

20oC 50oC

140oC 170oC

80oC 110oC

200oC 230oC

500

600

700

800

Wavelength(nm) Figure 8. Temperature-dependent emission spectra of CSMS host from 20 ℃ to 230 ℃ under the excitation of 270 nm.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Schematic explanation of luminescence process of host.

Luminescence process of CSMS might be agreed with the luminescence of host as depicted in Figure 9. Specifically, when host absorbed energy, the electrons were excited to the conduction band from the valence band, thus the cavities are stayed at the valence band (Process 1). The electronics of the conduction band will reach the bottom, yet, the cavities in the valence band rise to the top, which may be ascribed to the process of thermal equilibrium (Process 2 and 2’). And the electronics can be captured by the luminescence center (Process 3’). Then, the electrons on the conduction band bottom via the excited state and fall to ground state directly, which results in the fluorescence by compositing with vacancies (Process 3 and 4). Here, the long wave threshold (λe) about composite luminescence of electronic-hole can be estimated by formula

λ h

(2)

where h is Planck constant, c is the speed of light in vacuum, Eg is band gap width. Another luminescence process is about that the shallow trap capturing electronics. That is to say, the electrons on bottom of conduction band are captured by the shallow traps D1 (Process 5), and then these electrons transition come back to the conduction band because of the thermal disturbance. Finally, the electrons can composite with the luminescence center to emit light (Process 6). The third luminescence process about deep level capturing electronics is also mentioned. Deep energy level (D2) is away from the conduction band bottom so far and the electronics are always stayed on this level at room temperature. Through heating up, the electronics could jump into the conduction band from D2 and compositing with luminescence center to fluorescence if the


Page 12 of 34

Page 13 of 34

luminescence center can get back to the ground state without the non-radiation transitions (Process 7 and 8). A speculation about the luminescence mechanism of CSMS host might be interpreted by three light-emitting processes above. Host 0.001Eu2+ 0.005Eu2+ 0.01Eu2+ 0.03Eu2+ 0.06Eu2+

160000 140000

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120000 100000

(a)

Host 20000 0.01Mn2+ 2+ 0.05Mn 16000 0.07Mn2+ 2+ 0.10Mn 12000

(b)

80000

8000

60000 40000

4000

20000 0

0 50

100

150

o

200

250

300 50

100

10.

(a)

Thermoluminescence spectra

200

250

300

o

T ( C)

Figure

150

T ( C)

of

Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+

and

(b)

Ca0.75Sr0.2Mg1.05-y(Si2O6):yMn ；(c) Schematic diagram about the oxygen vacancies are filled up 2+

with Eu2+ and Mn2+ ions.

Figure 10a and 10b describe the thermoluminescence spectra of Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+ (x=0-0.06)

Ca0.75Sr0.2Mg1.05-y(Si2O6):yMn2+

and

(y=0-0.10),

respectively.

For

Ca0.75Sr0.2Mg1.05(Si2O6) host, two kinds of intrinsic traps circled in picture are shown, which is E1= 0.6453 eV, E2= 0.8271 eV by the thermoluminescence spectra and formula43,44

2.52 10.2 µ 0.42

ω

" !

2#

%$(3)

The parameters in equations (3) is explained as follows: E represents the trap depth, kB is Boltzmann’s constant; Tm is the temperature of the glow peak; ug is the geometrical form factor δ

and expressed as µg = (ω=δ+τ) , in which ω is the full width of half maximum,τ is half the width ω

toward low-temperature side, δ is the half-width toward high-temperature side. It is a reasonable assumption that the reason of these two different traps in the formation might be attributed to the special structure of [SiO3] and the space of oxygen among the different atoms. Therefore, the intensity of these traps are decreased monotonously after doping Eu2+ or Mn2+ ions and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

disappeared firstly when Eu2+ concentration was 0.06 while Mn2+ was 0.10. The reason of latter is that the ionic radius of Eu2+ is bigger than that of Mn2+.45-47 Schematic diagram that oxygen vacancies are filled up with Eu2+ and Mn2+ ions, as revealed in Figure 10c. With the doping of Eu2+ and Mn2+, in this picture, a part of atoms fill up the oxygen vacancies caused by [SiO3], whereas another part replace Ca or Mg atoms.48,49 And all the process above can result in the Vo becoming diminution, that is, the trap depth should be more shallower gradually and even vanishing.50,51 3.3. Luminescence properties of Ca0.75-xSr0.2Mg1.05-y(Si2O6):xEu2+, yMn2+. The emission (λex=270, 350 and 397 nm) and excitation (λem=453 nm) spectra of the Ca0.752+ 0.03Sr0.0.2Mg1.05(Si2O6):0.03Eu

sample are displayed in Figure 11. The intensity is the more

remarkable upon 350 nm excitation while the weakest at 270 nm in the emission spectra. In comparison to the emission spectra of CSMS (in Figure 7a), the peak of CSMS host at around 580 nm disappears completely under the excitation at 270 nm, which is mainly responsible for the original oxygen vacancies are filled up with Eu2+ ions. Moreover, there is no sharp Eu3+ emission peak around about 610 nm observed, which indicates the Eu3+ could be translated into Eu2+ completely. The excitation spectrum monitored at 453 nm present a wide band ranging from 200 to 450 nm, in which there are two obvious peaks around at 350 and 397 nm, corresponding to the 4f7-4f65d1 allowed transition of Eu2+ ions.24,52,53,57 Additionally, the broad extent of excitation spectrum would like to be interesting for application in n-UV white LEDs, which is usually desired. The profiles of emission spectra with relatively broad band ranging from 400 to 550 nm and the full-width at half-maximum(FWHM) is about 41 nm peaking at 453 nm originating from Eu2+ 4f65d1 - 4f7 with energy absorption from Eu2+ ions to CSMS host. The calculated Stokes shift between the excitation peak at 350 nm and emission peak at 453 nm is about 9709 cm-1. Specifically, ∆S and Γcan be linked by the s and h ω with following formula

∆S (2s 1)hω

(4)

Γ (T) 2.36hω√s.cos( hω2K T ) 3

(5)

where ∆S is Stokes shift, Γ represents FWHM, s is Huang-Rhys factor, h ω represents lattice phonon energy, KB is Boltzmann constant and T is temperature. Therefore, it is reasonable for


Page 14 of 34

Page 15 of 34

Stokes shift and FWHM in this study.

397nm

ex

=270nm

=350nm ex λ ex=397nm λ

4f 7

λ em=453nm

PLE 200

λ

4f 65d1

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250

300

350

PL 400

450

500

550

600

Wavelength(nm) Figure 11. Excitation and emission spectra of Ca0.75-0.03Sr0.2Mg1.05(Si2O6 ) :0.03Eu2+.

Figure 12. (a) Variation of emission spectra of Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+ (x=0.0010.08) excited with 350 nm and (b) the corresponding images of different concentration.

Figure 12(a) shows the emission spectra of Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+ phosphors upon 350 nm excitation with various Eu2+ concentration (x=0.001-0.08). One can note that a red-shift of emission spectra is observed, which is responsible for the bigger Eu2+ ions occupied the sites of Ca2+. Furthermore, the obtained experimental optimal concentration of Eu2+ in Ca0.752+ xSr0.2Mg1.05(Si2O6):xEu

is found to be 0.03 corresponding the photo 4. Photo 4 shows the best

brightness with the highest emission intensity, beyond which the emission intensity starts to decrease with increasing concentration of Eu2+ attributed to the concentration quenching effect as shown in Figure 12(b). For Ca2+ ions, however, it is accepted that concentration quenching occurs



because of the energy transfer among Eu2+ ions, whose probability increases as the concentration of Eu2+ increasing.[24] The critical distance (Rc) between activators such as Eu2+ here could be calculated by the following formula to further determine the energy transfer mechanism23,24 67

= 6

4 ≈ 2 89: ?(4@ )ℎνBC DEℎν F

x=0, y=0 x=0.03, y=0 x=0, y=0.25 x=0.08, y=0.25 x=0.03, y=0.15 65

Reflectance (R%)

60

(b)

50 40 35 30 200 242 300

400

Band gap =5.12 eV

45

CSMS:xEu2+, yMn2+ 300

CSMS Host

55

25

200

(7)

(a)

Reflectance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

400

500

600

700

800

Wavelength(nm)

600

700

800

Wavelength(nm) Figure 20. (a) Diffuse reflection spectra of Ca0.75Sr0.2Mg1.05(Si2O6):xEu2+, yMn2+. (b) Illustration of approximate band gap of CSMS host.



where h is Planck’s constant, υ is the frequency, Eg is the value of band gap, A is the proportional constant, n = 2 denotes the direct allowed transition. And F(R∞) is the Kubelka-Munk absorption coefficient calculated from the measured reflectance (R) through the following formula55-57

?(4@ )

(=GH" )

(8)

IH

Using the Kubelka-Munk function, the diffuse reflection spectrum was represent as (Rhυ)2 ~ hυ form,56 as shown in Figure 20b. According to the equation and reflection spectrum above, the optical band gap is evaluated approximately to be 5.12 eV (corresponding to the wavelength at 242 nm) for Ca0.75Sr0.2Mg1.05(Si2O6), which is critical due to the valence to conduction band transitions of Ca0.75Sr0.2Mg1.05(Si2O6). 2+

τ =0.78764 us

a1 1

2

3

4

5

τ= 0.72837us 0.65372us 0.60878us 0.55664us

x= 0.01 0.05 0.10 0.15

0.01Mn2+ 0.05Mn2+ 0.10Mn2+ 0.15Mn2+

2+

Eu

2+

0

n

a0

M ,y

Intensity(a.u.)

0.03Eu --Time

03 0.

1

2 2+ 3 4 Eu --Time(us)

5

6

Figure 21. Decay curves and calculated lifetimes of Eu2+ for CSMS:0.03Eu2+

0.72837 0.65372 0.60878 0.55664

Intensity(a.u.)

0.01 0.05 0.10 0.15

0.01Mn 2+ 0.05Mn2+ 0.10Mn2+ 0.15Mn2+

2+

Eu

( b 2) x =

0.01 0.05 0.10 0.15

τ= 4.15365ms 4.32719ms 8.74126ms 9.01213ms

0.01Mn 2+ 0.05Mn 2+ 0.10Mn 2+ 0.15Mn 2+

CSMS:yMn2+

M ,y

energy

n2

2 3 4 5 2+ Eu --Time(us)

6 5 10 15 20 25 30 35 40

x= 0.01 0.05 0.10 0.15

er

1

+

0

hi gh

Intensity(a.u.)

x= τ =

(b1)

03

Intensity(a.u.)

and Eu2+ in representative samples CSMS:0.03Eu2+, yMn2+.

0.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

Mn2+--Time(ms)

τ= 5.30515ms 13.15826ms 14.76863ms 15.02119ms

0.01Mn2+ 0.05Mn2+ 0.10Mn2+ 0.15Mn2+

0.03Eu2+, yMn2+

(b0)

10

20

30

40

50

60

70

80

Mn--Time(ms) Figure 22. Decay curves and calculated lifetimes of Eu2+ and Mn2+ in representative samples CSMS:0.03Eu2+, yMn2+.


(c1)

x=

τ =

0.00 0.01 0.05 0.10 0.15 0.20 0.25

6.75931 6.49817 6.03202 3.94617 3.76225 3.74496 3.72252

(c2)

0.00 0.01 0.05 0.10 0.15 0.20 0.25 2+

CSMS:yMn -Host

x=

τ=

0.01 0.05 0.07 0.10 0.15 0.20 0.25

4.15365 4.32719 5.65908 8.74126 9.01213 9.26814 9.32724

0.01 0.05 0.07 0.10 0.15 0.20 0.25 2+

2+

CSMS:yMn -Mn

energy

Mn2+

CSMS energy

5 10 15 20 25 30 35 40 3 6 9 12 15 18 21 24 27 2+ 2+ Time of host and Mn in CSMS:yMn samples Figure 23. Decay curves and calculated lifetimes of CSMS host and Mn2+ in representative CSMS:yMn2+ samples .

(d0)

Intensity(a.u.)

(d1)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity(a.u.)

Page 23 of 34

τ =6.75931ms

2

4

6

8

x=

τ =

0.001 0.005 0.01 0.03 0.06 0.08

4.65521 6.59193 7.7874 8.73228 9.71883 10.81789

10 12 14

CSMS--Time(ms)

4

8

0.001 0.005 0.01 0.03 0.06 0.08

12 16 20 24 28 32 36 2+ 2+ xEu , 0.25Mn ---CSMS Time(ms)

40

Figure 24. Decay curves and calculated lifetimes of host in CSMS:xEu2+, 0.25Mn2+ as well as the lifetime of CSMS host.

3.4. Lifetimes curves of CSMS:xEu2+, yMn2+ phosphors and energy transfer. The decay curves plotted in Figure 21 and Figure 22 show the decay properties of representative samples and to further confirm the energy transfer. Figure 21(a0) presents the decay curves and calculated lifetimes of Eu2+ in samples CSMS:0.03Eu2+ , yMn2+ as well as the lifetime of Eu2+ in 0.03Eu2+ single-doped sample is shown in Figure 21(a1). All the decay curves above are monitored at 453 nm and excited at 350 nm. The lifetime curves of Mn2+ in various CSMS:0.03Eu2+, yMn2+ samples and in Mn2+ single-doped with different concentration are displayed in Figure 22(b0) and (b2), respectively, which are both monitored at 680 nm under excitation at 415 nm. Besides, the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

lifetimes of Eu2+ in samples CSMS:0.03Eu2+, yMn2+ are shown in Figure 22(b1). All the decay curves are successfully fitted using the following two exponential equation45,56 P

J(K) JL D= MNO DI MNO (K/TI ) QR

(9)

where I (t) and I0 are the luminescence intensities at times t; A1, A2 and A3 are fitting constants and τ1, τ2 and τ3 are the decay times for the exponential components. Using these parameters, the average decay times (τ * ) can be determined by the formula as follows13,24

T∗

VR QR" WV" Q""

VR QR WV" Q"R

(10)

Calculating by the formula above, the lifetime of Eu2+ in sample 3%Eu2+ single-doped is 0.78764 us as shown in Figure 21(a1). However, the lifetimes of Eu2+ in CSMS:0.03Eu2+, yMn2+ are decreasing monotonically from 0.72837 us to 0.55664 us as shown in Figure 22(a0). Especially, when the content of Mn2+ is 0.01, the lifetime of Eu2+ decreased rapidly with respect to the sample of 3% Eu2+ single-doped. An increasing tendency for the lifetimes of Mn2+ single-doped compounds can be observed with the changing of Mn2+ concentration from 0.01 to 0.15. The lifetimes of Mn2+ in CSMS:0.03Eu2+, yMn2+ samples got longer compared with Mn2+ single-doped at the same concentration. This is an implication that energy transfer from Eu2+ to Mn2+ shows a good fit to our experimental results. The decay curves and calculated lifetime data of CSMS host and Mn2+ in representative CSMS:yMn2+ samples are presented in Figure 23. Note that here, in Figure 23(c1), the lifetimes of CSMS host in CSMS: yMn2+ monitored at 580 nm under excitation at 270 nm keep decreasing along with the increasing of Mn2+ concentration and the longest lifetime is τ =6.75931 ms in Mn2+ undoped CSMS. Thus, the lifetime changes significantly corresponded to concentration range of Mn2+ from 0.05 to 0.10, that is, the efficiency of energy transfer is the largest from CSMS host to Mn2+ in this range. Figure 23(c2) shows the decay curves and lifetime data of Mn2+ in CSMS:yMn2+ samples, in which the lifetime of Mn2+ increasing monotonically with the concentration augmenting. For Mn2+ single-doped, the concentration range from 0.05 to 0.10 with a remarkably increased of lifetime is consistent with the decay curves of host, which further to convince the energy transfer between CSMS host and Mn2+. The lifetime curves of host in CSMS:xEu2+, 0.25Mn2+ samples are shown in Figure 24(d1) as well as lifetime of itself is shown


Page 24 of 34

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in Figure 24(d0). The lifetime of host rises continuously with increasing of Eu2+ concentration (0.001-0.08), which can be attributed to the energy transfer from Eu2+ to CSMS host. Moreover, when the concentrations of Eu2+ are 0.001 and 0.005, the lifetimes of host in samples CSMS:xEu2+, 0.25Mn2+ are shorter than pure host duo to the effective energy transfer from Eu2+ to Mn2+ ions. Yet, energy from Eu2+ to host is weaker because the concentration of the Eu2+ is so little.

Figure 25. CIE chromaticity coordinates diagram of the as-prepared samples CSMS host; 0.1%Mn2+; 3%Eu2+; 3%Eu2+, 2%Mn2+ and the selected digital photo, respectively.

Table 2. The variation of CIE chromaticity coordinates (x, y) and QY (%) for Ca0.75-xSr0.2Mg1.05(Si2O6):xEu2+ phosphors excited at 350 nm UV radiation. Samples

CSMS:xEu2+

1

x=0.001

(0.1534, 0.0865)

66.08

2

x=0.005

(0.1543, 0.0919)

56.64

3

x=0.01

(0.1491, 0.0978)

54.34

4

x=0.03

(0.1451, 0.0736)

51.53

5

x=0.06

(0.1429, 0.0755)

31.24

6

x=0.08

(0.1449, 0.0773)

23.25

CIE (x, y)

QY (%)

3.5. CIE chromaticity coordinates, quantum yields. A series of Eu2+ single doped and Eu2+/Mn2+ co-doped CSMS samples have been prepared to tune the emission colors, which results in the color changed from blue to blue-violet emitting with adjusting the concentration ratio of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

Mn2+ ions to constant Eu2+ concentration, which is shown in Table 2 and Table 3. The representative samples are displayed in Figure 25 with toning from the typical yellow to bluewhite. Otherwise, the measured values of quantum yields of as-prepared samples are also listed, from which we can see that the maximum quantum yield in as-prepared samples of Eu2+ singledoped can reach 66.08%. The quantum yield is decreased significantly after adding Mn2+ ions into CSMS: 3%Eu2+ samples, which indicates that Mn2+ ions have a significantly effect on the quantum yield by influencing the particle morphology, size, and crystalline defects. Table 3. The variation of CIE chromaticity coordinates (x, y) and QY (%) for Ca0.752+ 2+ 0.03Sr0.2Mg1.05-y(Si2O6):3%Eu ，yMn phosphors

excited at 356 nm UV radiation.

Samples

CSMS:3%Eu2+,yMn2+

1

y=0.01

(0.1818,0.0999)

24.62

2

y=0.02

(0.2682,0.2056)

21.52

3

y=0.03

(0.2126,0.1747)

20.13

4

y=0.05

(0.2295,0.1762)

16.83

5

y=0.08

(0.2301,0.1515)

16.49

6

y=0.10

(0.2317,0.1346)

10.57

7

y=0.15

(0.2365,0.1298)

11.02

8

y=0.20

(0.2489,0.1203)

21.94

CIE (x, y)

QY (%)

4. CONCLUSIONS As stated before, a series of novel Ca0.75-xSr0.2Mg1.05-y(Si2O6):xEu2+, yMn2+ phosphors were synthesized successfully by the traditional high temperature solid-state reaction in reductive atmosphere. The luminescence properties, luminescence mechanism and energy transfer of phosphors

were

characterized

by

the

X-ray

diffraction

(XRD),

diffuse

reflection,

thermoluminescence glow curves (TL), photoluminescence spectra (PL), Temperature-dependent PL spectra, decay curves, scanning electron micrograph (SEM), electron-dispersive X-ray (EDX), PL quantum yields (QYs) fluorescent photographs and CIE chromaticity coordinates. The


Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compound has the structure of monoclinic system with special structure of [SiO3]. The emission spectra of CSMS host present a broad emission band from 350 to 800 nm under the excitation of 270 nm, which shows two distinct emission peaks, 453 nm and 580 nm, respectively. The two peaks are correspond to the different trap depths (E1=0.6453 eV, E2=0.8271 eV) and these traps are originated from the Vo native defects, which resulted in the orange-yellow emitting of the CSMS host. The luminescence mechanism of host depended on oxygen vacancy is proved by temperature-dependent emission spectra and thermoluminescence spectra. For Eu2+ single-doped, these phosphor samples have an obvious absorption in the near-ultraviolet and blue region with a wide band of excitation spectrum ranging from 200 to 450 nm monitored at 453 nm. Moreover, the emission spectrum show an obvious red-shift phenomenon, which is derived from the bigger Eu2+ occupied the site of Ca2+. Besides, we can observe that the critical quenching concentration of Eu2+ was about 0.03. The theory that Eu2+ and Mn2+ ions fill up the oxygen vacancies is also mentioned in paper. The Vo are filled up entirely when Eu2+ concentration was 0.06 while Mn2+ was 0.10, which can be ascribed to the difference of ionic radius and the latter is smaller. The energy transfer from Eu2+ to Mn2+, Eu2+ to host as well as host to Mn2+ has been demonstrated by photoluminescence spectra and decay curves, which is presented in Eu2+/Mn2+ co-doped samples. The maximum quantum yield in as-prepared Eu2+ sing-doped samples can reach 66.08% and typical CIE chromaticity coordinates is (0.1451,0.0736). Moreover, there is a blue-white emitting (0.2682, 0.2056) in sample CSMS:3%Eu2+,2%Mn2+ with the energy transfer from Eu2+ to Mn2+. The results above indicated that as a kind of new materials to emit an orange-yellow itself, the CSMS host represents a good application prospect in field of phosphors. AUTHOR INFORMATION Corresponding Author *[email protected] (Panlai Li); Tel: +86 312 5977068. *[email protected] (Zhijun Wang); Tel: +86 312 5977068. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS



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Page 28 of 34

The work is supported by China Postdoctoral Science Foundation funded project (No.2015M58131), the Funds for Distinguished Young Scientists of Hebei Province, China (No.A2015201129),

the

Natural

Science

Foundation

of

Hebei

Province,

China

(Nos.A2014201035, E2014201037), the Education Office Research Foundation of Hebei Province, China (Nos.ZD2014036, QN2014085), the National Natural Science Foundation of China (No.50902042), the Midwest Universities Comprehensive Strength Promotion Project. REFERENCES (1) Shang, M. M.; Li, C. X.; Lin, J. How to Produce White Light in A Single-Phase Host? Chem. Soc. Rev. 2014, 43, 1372-1386. (2) Xu, S. C.; Li, P. L.; Wang, Z. J.; Li, T.; Bai, Q. Y.; Sun, J.; Yang, Z. P. Luminescence and Energy Transfer of Eu2+/Tb3+/Eu3+ in LiBaBO3 Phosphors with Tunable-Color Emission. J. Mater. Chem. C 2015, 3, 9112-9121. (3) With, D. G. Luminescence Properties of Eu2+-Activated Alkaline-Earth Silicon-Oxynitride MSi2O2-DeltaN2+2/3Delta (M = Ca, Sr, Ba): A Promising Class of Novel LED Conversion Phosphors. Chem. Mater. 2005, 17, 3242-3248. (4) Krishnan, R.; Thirumalai, J. Correction: Up/Down Conversion Luminescence Properties of (Na0.5Gd0.5)MoO4:Ln3+ (Ln = Eu, Tb, Dy, Yb/Er, Yb/Tm, and Yb/Ho) Microstructures: Synthesis, Morphology, Structural and Magnetic Investigation. New J. Chem. 2014, 38, 3480-3491. (5) Liu, X. M.; Li, C. X.; Quan, Z. W.; Cheng, Z. Z.; Lin, J. Tunable Luminescence Properties of CaIn2O4:Eu3+ Phosphors. J. Phys. Chem. C 2007, 111, 16601-16607. (6) Wang, Y. H.; Brik, M. G.; Dorenbos, P.; Huang, Y.; Tao, Y.; Liang, H. B. Enhanced Green Emission of Eu2+ by Energy Transfer from The 5D3 Level of Tb3+ in NaCaPO4. J. Phys. Chem. C 2014, 118, 7002-7009. (7) Jou, J. H.; Kumar, S.; Agrawal, A.; Li, T. H.; Sahoo, S. Approaches for Fabricating High Efficiency Organic Light Emitting Diodes. J. Mater. Chem. C 2015, 1, 2974-3002. (8) Volker, B.; Cees, R.; Andries, M. Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce. Chem. Mater. 2009, 21, 2077-2084. (9) Liu, B. T.; Wang, Y. H.; Wen, Y.; Zhang, F.; Zhu, G.; Zhang, J. Photoluminescence Properties of S-Doped BaAl12O19:Mn2+ Phosphors for Plasma Display Panels. Mater. Lett. 2012, 75, 137-139.


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(10) Hao, Z. D.; Zhang, J. H.; Zhang, X.; Sun, X. Y.; Luo, Y. S.; Lu, S. Z.; Wang, X. J. White Light Emitting Diode by Using-Ca2P2O7:Eu2+, Mn2+ Phosphor. Appl. Phys. Lett. 2007, 90, 261113-1-261113-3. (11) Zhang, Y.; Geng, D. L.; Shang, M. M.; Wu, Y.; Li, X. J.; Lian, H. Z.; Cheng, Z. Z.; Lin, J. Single ‐ Composition Trichromatic White ‐Emitting Ca9MgNa(PO4)7:Ce3+/Tb3+/Mn2+ Phosphors– Soft Chemical Synthesis, Luminescence, and Energy-Transfer Properties. Eur. J. Inorg. Chem. 2013, 2013, 4389-4397. (12) Wang, X. J.; Jia, D. D.; Yen, W. M. Mn2+ Activated Green, Yellow, and Red Long Persistent Phosphors. J. Lumin. 2003, 102, 34-37. (13) Li, K.; Xu, M. J.; Fan, J.; Shang, M. M.; Lian, H. Z.; Lin, J. Tunable Green to YellowishOrange Phosphor Na3LuSi2O7:Eu2+, Mn2+ via Energy Transfer for UV-LEDs. J. Mater. Chem. C 2015, 3, 11618-11628. (14) Li, P. L.; Wang, Z. J.; Yang, Z. P.; Guo, Q. L. A Novel, Warm, White Light-Emitting Phosphor Ca2PO4Cl:Eu2+, Mn2+ for White LEDs. J. Mater. Chem. C 2014, 2, 7823-7829. (15) Bian, L.; Liu, C. W.; Gao, J.; Jing, X. P. Energy Transfer and Color Tuning in The Ba9Sc2Si6O24:Ce3+, Eu2+, Mn2+ Phosphor. RSC Advances 2015, 5, 69458-69465. (16) Liu, W. R.; Huang, C. H.; Yeh, C. W.; Tsai, J. C.; Chiu, Y. C.; Yeh, Y. T.; Liu, R. S. A Study on The Luminescence and Energy Transfer of Single-Phase and Color-Tunable KCaY(PO4)2:Eu2+, Mn2+ Phosphor for Application in White-Light LEDs. Inorg. Chem. 2012, 51, 9636-9641. (17) Ma, L.; Wang, D. J.; Zhang, H. M.; Gu; T. C.; Yuan, Z. H. The Origin of 505 nm-Peaked Photoluminescence from Ba3MgSi2O8:Eu2+, Mn2+ Phosphor for White-Light-Emitting Diodes. ECS. Lett. 2008, 11, E1-E4. (18) Miao, S. H.; Xia, Z. G.; Zhang, J.; Liu, Q. L. Increased Eu2+ Content and Codoping Mn2+ Induced Tunable Full-Color Emitting Phosphor Ba1.55Ca0.45SiO4:Eu2+, Mn2+. Inorg. Chem. 2014, 53, 10386-10393. (19) Xie, R. J.; Hirosaki, N.; Sakuma, K.; Kimura, N. White Light-Emitting Diodes (LEDs) Using (Oxy) Nitride Phosphors. J. Phys. D Appl. Phys. 2008, 41, 2123-2131.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

(20) Lin, C. C.; Liu, R. S. Advances in Phosphors for Light-Emitting Diodes. J. Phys. Chem. Lett. 2011, 2, 1268-1277. (21) Liu, Y.; Zhang, X.; Hao, Z.; Wang, X.; Zhang, J. Tunable Full-Color-Emitting Ca3Sc2Si3O12:Ce3+, Mn2+ Phosphor via Charge Compensation and Energy Transfer. Chem. Commun. 2011, 47, 10677-10679. (22) Jia, Y. C.; Huang, Y. J.; Zheng, Y. H.; Guo, N.; Qiao, H.; Zhao, Q.; Lv, W. Z.; You, H. P. Color Point Tuning of Y3Al5O12:Ce3+ Phosphor via Mn2+–Si4+ Incorporation for White Light Generation. J. Mater. Chem. 2012, 22, 15146-15152. (23) Li, X.; Choy, W. C.; Xie, F.; Zhang, S.; Hou, J. Room-Temperature Solution-Processed Molybdenum Oxide as A Hole Transport Layer with Ag Nanoparticles for Highly Efficient Inverted Organic Solar Cells. J. Mater. Chem. A 2013 , 1, 6614-6621. (24) Li, K.; Xu, J.; Cai, X. C.; Fan, J.; Zhang, Y.; Shang, M. M.; Lian, H. Z.; Lin, J. An Efficient Green-Emitting

α-Ca1.65Sr0.35SiO4:Eu2+

Phosphor

for

UV/n-UV

W-LEDs:

Synthesis,

Luminescence and Thermal Properties. J. Mater. Chem. C 2015, 3, 6341-6349. (25) Jiao, H.; Y.; Wang, Y. H. A Potential Red-Emitting Phosphor CaSrAl2SiO7:Eu3+ for NearUltraviolet light-Emitting Diodes. Phys B: Condens. Matter 2012, 407, 2729-2733. (26) Benna, P.; Chiari, G.; Bruno, E. Structural Modifications in Clinopyroxene Solid Solutions: The Ca-Mg and Ca-Sr Substitutions in The Diopside Structure. Miner. Petrol. 1987, 36, 71-84. (27) Liu, Z. S.; Jing, X.; Wang, L. Luminescence of Native Defects in Zn2GeO4. J. Electrochem. Soc. 2007, 154, H500-H506. (28) Lv, W. Z.; Jiao, M. M.; Zhao, Q.; Shao, B. Q.; L, W.; You, H. P. Ba1.3Ca0.7SiO4:Eu2+, Mn2+: A Promising Single-Phase, Color-Tunable Phosphor for Near-Ultraviolet White-Light-Emitting Diodes. Inorg. Chem. 2014, 53, 11007-11014. (29) Dirksen, G. J.; Blasse, G. Tetracalcium Gadolinium Oxoborate (Ca4GdO(BO3)3) as A New Host Lattice for Luminescent Materials. J. Cheminformatics 1993, 24, 121-126. (30) Zhang, Y. Y.; Xia, Z. G.; Wu, W. W. Preparation and Luminescence Properties of Blue‐ Emitting Phosphor Ba2Ca(PO4)2:Eu2+. J. Am. Ceram. Soc. 2013, 96, 1043-1046.


Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Yu, H.; Deng, D. G.; Zhou, D. T.; Yuan, W.; Zhao, Q. E.; Hua, Y. J.; Zhao, S. L.; Huang, L. H.; Xu, S. Q. Ba2Ca(PO4)2:Eu2+ Emission-Tunable Phosphor for Solid-State Lighting: Luminescent Properties and Application as White Light Emitting Diodes. J. Mater. Chem. C 2013, 1, 5577-5582. (32) Xia, Y.; Chen, J.; Liu, Y. G.; Molokeev, M. S.; Guan, M.; Huang, Z.; Fang, M. Crystal Structure Evolution and Luminescence Properties of Color Tunable Solid Solution Phosphors Ca2+xLa8−x(SiO4)6−x(PO4)xO2:Eu2+. Dalton Trans. 2016, 45, 1007-1015. (33) Garces, N. Y.; Wang, L.; Bai, L.; Giles, N. C. Role of Copper in The Green Luminescence from ZnO Crystals. Appl. Phys. Lett. 2002, 81, 622-624. (34) Liu, R. S.; Wang, W. N.; Chang, C. T.; Wu, P. T. Synthesis and Characterization of High-Tc Superconducting Oxides by The Modified Citrate Gel Process. Jpn. J. Appl. Phys. 1989, 28, L2155-L2157. (35) Albakry, M.; Guazzato, M.; Swain, M. V. Biaxial Flexural Strength, Elastic Moduli, and XRay Diffraction Characterization of Three Pressable All-Ceramic Materials. J. Prosthet. Dent. 2003, 89, 374-380. (36) Ravikumar, B. S.; Nagabhushana, H.; Sunitha, D. V.; Sharma, S. C.; Nagabhushana, B. M.; Shivakumara, C. Plant Latex Mediated Green Synthesis of ZnAl2O4:Dy3+(1–9mol%) Nanophosphor for White Light Generation. J. Alloy. Compd. 2014, 585, 561-571. (37) Li, W.; Lee, J. Microwave-Assisted Sol−Gel Synthesis and Photoluminescence Characterization of LaPO4:Eu3+, Li+ Nanophosphors. J. Phys. Chem. C 2008, 112, 11679-11684. (38) Kumar, M.; Gupta, S. K. An Insight into Optical Spectroscopy of Intense Green Emitting ZnAl2O4:Tb3+ Nanoparticles: Photo, Thermally Stimulated Luminescence and EPR Study. J. Lumin. 2015, 168, 151-157. (39) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gande, B. E. Mechanisms behind Green Photoluminescence in ZnO Phosphor Powders. J. Appl. Phys. 1996, 79, 7983-7990. (40) Xie, T.; Guo, H. X.; Zhang, J. Y.; He, Y. N.; Lin, H.; Chen, G. L.; Zheng, Z. S. Effects of Oxygen Vacancies on Luminescent Properties of Green Long-Lasting Phosphorescent (LLP) Material α-Zn3(PO4)2:Mn2+, K+. J. Lumin. 2015, 170, 150-154.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(41) Robbins, D. J.; Cockayne, B.; Lent, B.;Duckworth, C. N.; Glasper, J. L. Investigation of Competitive Recombination Processes in Rare-Earth Activated Garnet Phosphors. Phys. Rev. B 1979, 19, 1254-1269. (42) Robbins, D. J.; Cockayne, B.; Glasper, J. L.; Lent, B. The Temperature Dependence of Rare‐ Earth Activated Garnet Phosphors II. A Comparative Study of Ce3+, Eu3+, Tb3+, and Gd3+ in Y3Al5O12. J. Electrochem. Soc. 1979, 126, 1221-1228. (43) Chen, R. Glow Curves with General Order Kinetics. J. Electrochem. Soc. 1969, 116, 12541257. (44) Jin, Y. H.; Hu, Y. H.; Chen, L.; Wang, X. J. Luminescence Properties of Long Persistent Phosphors BaZrSi3O9:R3+(R=Eu, Sm, Dy, Tb and Pr) Based on Host Sensitization. Opt. Mater. 2014, 36, 1814-1818. (45) Eeckhout, K. V. D.; Smet, P. F.; Poelman, D. Persistent Luminescence in Eu2+-Doped Compounds: A Review. Mater. 2010, 3, 2536-2566. (46) Sohn, K. S.; Seo, S. Y.; Park, H. D.; Sohn, K. S.; Seo, S. Y.; Park, H. D. Search for Long Phosphorescence Materials by Combinatorial Chemistry Method. Electrochem. Solid-State. Lett. 2001, 4, H26-H29. (47) Lin, C. C.; Xiao, Z. R.; Guo, G. Y.; Chan, T. S.; Liu, R. S. Versatile Phosphate Phosphors ABPO4 in White Light-Emitting Diodes: Collocated Characteristic Analysis and Theoretical Calculations. J. Am. Chem. Soc. 2010, 132, 3020-3028. (48) Song, F. L.; Chen, D. H.; Yuan, Y. H. Synthesis of Sr2MgSi2O7:Eu, Dy and Sr2MgSi2O7:Eu, Dy, Nd by A Modified Solid-State Reaction and Their Luminescent Properties. J. Alloy. Compd. 2008, 458, 564-568. (49) Hölsä, J.; Aitasalo, T.; Jungner, H.; Lastusaari, M.; Niittykoski, J.; Spano, G. Role of Defect States in Persistent Luminescence Materials. J. Alloy. Compd. 2004, 374, 56-59. (50) Secu, C. E.; Secu, M.; Ghica, C.; Mihut, L. Rare-Earth Doped Sol–Gel Derived Oxyfluoride Glass–Ceramics: Structural and Optical Characterization. Opt. Mater. 2011, 33, 1770-1774. (51) Jayaramaiah, J. R.; Lakshminarasappa, B. N.; Nagabhushana, B. M. Luminescence Studies of Europium Doped Yttrium Oxide Nano Phosphor. Sensor. Actuat. B Chem. 2012, 173, 234-238.


Page 32 of 34

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(52) Tsai, Y. T.; Chiang, C. Y.; Zhou, W.; Lee, J. F.; Sheu, H. S.; Liu, R. S. Structural Ordering and Charge Variation Induced by Cation Substitution in (Sr, Ca) AlSiN3:Eu Phosphor. J. Am. Chem. Soc. 2015, 137, 8936-8939. (53) Li, T.; Li, P. L.; Wang, Z. J.; Xu, S. C.; Bai, Q. Y.; Yang, Z. P. Structure, Luminescence Properties and Energy Transfer of Tb3+–Eu3+ Codoped LiBaB9O15 Phosphors. Dalton Trans. 2015, 44, 16840-16846. (54) Xin, M.; Tu, D. T.; Zhu, H. M.; Luo, W. Q.; Liu, Z. G.; Huang, P.; Li, R. F.; Cao, Y. G.; Chen, X. Y. Single-Composition White-Emitting NaSrBO3:Ce3+, Sm3+, Tb3+ Phosphors for NUV Light-Emitting Diodes. J. Mater. Chem. C 2015, 3, 7286-7293. (55) Yamashita, N. Luminescence Centers of Ca(S:Se) Phosphors Activated with Impurity Ions Having s2 Configuration. I. Ca(S:Se):Sb3+ Phosphors. J. Phys. Soc. Jpn. 1973, 35, 247-250. (56) Chen, X.; Xia, Z. G.; Liu, Q. L. Synthesis, Structure and Luminescence Properties of New Chloro-Germanate Phosphors Ca₃GeO₄Cl₂:Eu²+. J. Luim. 2014, 130, 1818–1824. (57) Lee, S. P.; Liu, S. D.; Chan, T. S.; Chen, T. M. Synthesis and Luminescence Properties of Novel Ce3+-and Eu2+-Doped Lanthanum Bromothiosilicate La3Br(SiS4)2 Phosphors for White LEDs. ACS Appl. Mater. Interfaces. 2016, 8, 9218-9223.



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Tunable Emission Phosphor Ca0.75Sr0.2Mg1.05(Si2O6):Eu2+, Mn2+:

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